This module provides different functions to sign a Pass using Apple's signpass
command-line tool.

I intend to move the signing process to a Haskell native OpenSSL binding later
on, but due to time constraints didn't get around to it yet.

If you want to use this module with an existing .pkpass file, you can import its
pass.json file using the function loadPass.

The function signpass creates a random UUID during the signing process, uses this UUID
as the passes' serial number and returns it along with the path to the signed pass.

The funciton signpassWithModifier creates a random UUID during the signing process,
passes this UUID on to a function that modifies the pass accordingly (e.g. sets the serial
number or the barcode payload to the UUID) and otherwise works like signpass.
The function updateBarcode is provided as well.

The function, signpassWithId, takes an existing ID and signs the pass
using that for the serial number and file name. This will most likely be used
for updating existing passes.

Using the function is very simple, assuming you have created a Pass called
myPass and you have the related assets (e.g. the logo.png and icon.png files)
stored in a folder named myPass/.

You want the signed pass to be stored in a folder called passes/. You call
signpass like this:

(path, passId) <- signpass "myPass" "passes" myPass

You will find the pass at path with the filename passId.pkpass. Using the
types from Passbook.Types ensures that passes are generated correctly.

The signpass utility must have access to your Keychain to be able to retrieve
your Pass Type Certificate for the pass to sign. You should run it once outside
of any program to grant it full access to your Keychain.

Please note that an icon.png file must be present in your asset folder,
otherwise the generated pass will not work.

Refer to Apple's Passbook documentation at https://developer.apple.com/passbook/
for more information or to retrieve the signpass tool which is included in the
Passbook Support Materials. (iOS Developer Membership necessary)

Works like signpass, except for the fourth argument which is a
modifier function that updates the pass with the generated UUID.
This is useful for cases where you want to store the UUID in the barcode
or some other field on the pass as well.